(1) Field of the Invention
The invention relates to a method for heating of a pre-warmed press muffle used for dental ceramics in a press furnace, as well as a control device and a furnace, which includes such a control device.
(2) Description of Related Art
So far it was assumed that input of a target temperature, a heating rate and a relative long holding time on the target temperature sufficiently defines the method for heating of a press muffle used for dental ceramics, before the pressing operation, as part of a press program starts. The input temperature is held constant and coincides more or less accurately with the actual temperature in the press muffle (also shortly named “muffle” in the following), depending on the type of furnace. This method is schematically shown in FIG. 2. The process specifications of the ceramics manufacturers warn of over-heating and refer to the coincidence of the prescribed temperatures in the process. Consequently, as specified above, the only usual method since many years is to control the press furnaces by adjusting the target temperature as maximum temperature and to heat-up the entire muffle slowly to this temperature. Since embedding masses, forming the muffles exhibit a very low thermal conductivity, long heating times are required. Nevertheless, the actual temperature inside the muffle is still up to 30° C. below the adjusted target temperature on beginning of the pressing operation. The indication of the processing temperature of the ceramic is usually adapted to this problem and has a higher value than actually required. Tests and measurements within the muffles evidenced this.
When defective pressing (the form is incompletely filled) happens, it is often attempted to extend the holding time or to correct the target temperature to a higher level. However, both measures change nothing in the fact that a temperature gradient exists within the muffles from the outside to the inside (e.g. up to 60° C. in a muffle having 300 g). Thus, defective pressing by over-heating or early solidification of the ceramic on the path into the form often occurs. Good pre-warming can be achieved with muffles having 100 g embedding mass (diameter 38 mm). However, for more complex works such as multiple models the use of muffle forms for 200 g embedding mass (diameter 48 mm) is required. Even larger works (e.g. multi-element dental bridges) were so far excluded of the technology of the ceramics pressing. On the one hand, this is to the missing indication of existing ceramic materials (large bridge constructions require a high breaking strength), on the other hand because of the problem to heat the required muffles with 300 g (diameter 62 mm) in a homogeneous manner. A new technology (press-to-metal), which is used more frequently, eliminates the problem of flexural strength of dental ceramics. In this method a frame of a dental metal alloy serves as basis for the press ceramics. While this technology is established since more than 50 years in dentistry by applying the ceramic by subsequent layers, the press-to-metal method is substantially more effective to create high-quality artificial dentures. In the future, this method requires uniform heating, in particular of larger press muffles.
Many dental technicians consider ceramics work of high complexity as “not feasible”. Hardly a work-piece succeeds without traces of material damages or discolorations. In addition, high cost and urgency oppose the required long program times of the heating phase. Even with small muffles the temperature is often not exact and uniform enough, so that in particular bright dental colours become greyish. Quality problems often lead to repetition of work or expensive repairs.